net: ipv6: Free received NA net_buf

If we receive a neighbor advertisement, we need to free its
net_buf because we are returning NET_OK to the caller. This
return code means that we consumed the net_buf but we did
not call net_nbuf_unref() in this case.

Change-Id: Ia6d8f1b440be87eff5d2b14a23336a37be6d6a04
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>